Corresponding control methods for operating once-through steam generators, and in particular once-through steam generators which are configured as heat recovery steam generators, are known from WO 2009/068446 A2 and WO 2009/150055 A2. In the method described here, a setpoint value for the feed water mass flow rate to be adjusted will be specified for a once-through steam generator having an evaporator heating surface.
Particularly in vertical heat recovery steam generators, for thermodynamic optimization of the steam generator circuit, evaporators may be used which are separated into two or more evaporator heating surfaces in the flue gas channel because of at least one interposed intermediate heating surface not belonging to the evaporator. This means in practice that, in relation to its path from the evaporator inlet to the evaporator outlet, the flue gas must pass additional heating surfaces at which it is likewise cooled. The flue gas temperature is therefore reduced from the inlet to the outlet of the evaporator not exclusively only by the heat power transferred to a flowing fluid in the evaporator heating surfaces, but also by the heat power transferred in the intermediate heating surfaces. If, however, the predictive control of the feed water mass flow rate known from WO 2009/068446 A2 or WO 2009/150055 A2 is then used in such steam generator circuits with interposed intermediate heating surfaces, excessively high heat absorption is determined for the evaporator since the flue gas has also released heat to the interposed heating surfaces but this release of heat is not evaluated and taken into account. As a result, an excessively high setpoint value will thus be calculated for the feed water mass flow rate.